1. Field of Invention
This invention relates to a process for producing biodiesel alkyl esters from biological feedstock materials wherein a flowing system containing a supercritical fluid and an alcohol at elevated temperature and pressure is used to extract biological fats and oils from biomass feedstock and transport the extracted material directly into a reaction chamber that is maintained at a separate reaction temperature wherein a transesterification reaction is carried out between the extracted fats and oils and the alcohol with or without the involvement of a separate catalyst and with or without the introduction into the reaction chamber of additional alcohol. A typical supercritical fluid used would be carbon dioxide, and a typical alcohol used would be methanol or ethanol. The extraction step and the reaction step are carried out with the supercritical fluid and alcohol solution pressurized and heated to a supercritical or subcritical state. According to the present invention, an improvement to biodiesel preparation is afforded by the simplification of the extraction and reaction processes into a single flowing system. Also according to the invention, a fatty acid alkyl ester product which can be effectively utilized as a biodiesel fuel substance can be efficiently prepared directly from biomass inputs without the need for prior extraction or other oil isolation processes.
2. Description of Prior Art
Biodiesel fuel, consisting of alkyl esters of fatty acids, is most commonly produced from oilseed feedstock through a series of disconnected steps. Typically the plant triglycerides are first isolated by crushing and grinding the seed and then expeller pressing the pulverized seed to release the oils. This intermediate product is then processed to dry it and remove free fatty acids. Then the purified oil is utilized in a transesterification reaction with a short chain alcohol such as methanol, along with a solution phase acid or base catalyst, typically a strong base such as sodium hydroxide, to facilitate the reaction and heated for several hours to form alkyl esters of the fatty acids and glycerol. Then the fatty acid alkyl esters are isolated and further purified to yield a biodiesel fuel.
In attempts to improve on the reaction rates, yields, and the purity of the products, it has been determined that high pressure, high temperature methanol can be used without an added solution phase catalyst in the transesterification of isolated vegetable oil (7, 13). Notably, this reaction is relatively insensitive to the presence of water compared to the base or acid-catalyzed transesterification reactions most frequently used (9). Research groups at many locations around the world have published or otherwise disclosed transesterification reactions of triglycerides using supercritical methanol as a reaction medium and reactant (1-8, 10-22).
The prior art clearly teaches that biodiesel fatty acid alkyl esters can be formed by the transesterification of vegetable oils with alcohols in supercritical fluids. In the prior art, the supercritical fluids are either composed of an alcohol such as methanol or are composed of a mixture of a nonreactive critical fluid and an alcohol such as methanol as the transesterification reactant; prior art teaches that these reactions have been induced to occur with or without an added catalyst (7).
For example, Nakayama, et al., (U.S. Pat. No. 6,960,672) teaches that a transesterification reaction between fats or oils and an alcohol in a supercritical or subcritical state can be used in the presence of a alkaline metal oxide, hydroxide, or carbonate catalyst to form fatty acid alkyl esters suitable for utilization as a biodiesel fuel.
Further, Han, et al., teaches that the supercritical methanol transesterification of soybean oil can by enhanced by the addition of carbon dioxide to the reaction mixture (8).
Further, Ginosar and Fox (U.S. Pat. No. 6,887,283) teaches a process to form a biodiesel fuel or lubricant comprising dissolving streams of a glyceride or free fatty acid containing substance in a stream of a critical fluid with or without a short chain alcohol cosolvent, combining this solution with a stream of a short chain alcohol or water, then reacting this mixture over a solid or liquid acidic or basic catalyst to form an alkyl ester and glycerol, then separating out the glycerol, then separating out the alkyl ester from the critical fluid medium, then recycling the critical fluid medium for use in a later reaction.
All prior art teaches the use of purified fats and oils as the feedstock for the transesterification reaction. Notably, none of the prior art anticipates the use of a supercritical solution to extract fats and oils from a solid vegetable feedstock and then utilize the extracted material directly by passing it from the extraction vessel into a reaction zone, solving the problem of needing to start with isolated oils and fats as a reactant. This inventive step allows for significant energy, time, and cost savings in the present invention.